This invention is directed to a novel antenna structure including an antenna array having a power amplifier chip operatively coupled to, and in close proximity to each antenna element in the antenna array.
In communications equipment such as cellular and personal communications service (PCS), as well as multi-channel multi-point distribution systems (MMDS) and local multi-point distribution systems (LMDS) it has been conventional to receive and retransmit signals from users or subscribers utilizing antennas mounted at the tops of towers or other structures. Other communications systems such as wireless local loop (WLL), specialized mobile radio (SMR) and wireless local area network (WLAN) have signal transmission infrastructure for receiving and transmitting communications between system users or subscribers which may also utilize various forms of antennas and transceivers.
All of these communications systems require amplification of the signals being transmitted and received by the antennas. For this purpose, it has heretofore been the practice to use a conventional linear power amplifier system, wherein the typical expense of providing the necessary amplification is typically between U.S. $100 and U.S. $300 per watt in 1998 U.S. dollars. In the case of communications systems employing towers or other structures, much of the infrastructure is often placed at the bottom of the tower or other structure with relatively long coaxial cables connecting with antenna elements mounted on the tower. The power losses experienced in the cables may necessitate some increase in the power amplification which is typically provided at the ground level infrastructure or base station, thus further increasing expense at the foregoing typical costs per unit or cost per watt.
Moreover, conventional power amplification systems of this type generally require considerable additional circuitry to achieve linearity or linear performance of the communications system. For example, in a conventional linear amplifier system, the linearity of the total system may be enhanced by adding feedback circuits and predistortion circuitry to compensate for the nonlinearities at the amplifier chip level, to increase the effective linearity of the amplifier system. As systems are driven to higher power levels, relatively complex circuitry must be devised and implemented to compensate for decreasing linearity as the output power increases.
Output power levels for infrastructure (base station) applications in many of the foregoing communications systems is typically in excess of ten watts, and often up to hundreds of watts which results in a relatively high effective isotropic power requirement (EIRP). For example, for a typical base station with a twenty watt power output (at ground level), the power delivered to the antenna, minus cable losses, is around ten watts. In this case, half of the power has been consumed in cable loss/heat. Such systems require complex linear amplifier components cascaded into high power circuits to achieve the required linearity at the higher output power. Typically, for such high power systems or amplifiers, additional high power combiners must be used.
All of this additional circuitry to achieve linearity of the overall system, which is required for relatively high output power systems, results in the aforementioned cost per unit/watt (between $100 and $300).
The present invention proposes distributing the power across multiple antenna (array) elements, to achieve a lower power level per antenna element and utilize power amplifier technology at a much lower cost level (per unit/per watt).
In accordance with one aspect of the invention, power amplifier chips of relatively low power and low cost per watt are utilized in a relatively low power and linear region in an infrastructure application. In order to utilize such relatively low power, low cost per watt chips, the present invention proposes use of an antenna array in which one relatively low power amplifier chip is utilized in connection with each antenna element of the array to achieve the desired overall output power of the array.
Accordingly, a relatively low power amplifier chip typically used for remote and terminal equipment (e.g., handset or user/subscriber equipment) applications may be used for infrastructure (e.g., base station) applications. In accordance with the invention, the need for distortion correction circuitry and other relatively expensive feedback circuits and the like used for linear performance in relatively high power systems is eliminated. The linear performance is achieved by using the relatively low power chips within their linear output range. That is, the invention proposes to avoid overdriving the chips or requiring operation close to saturation level, so as to avoid the requirement for additional expensive and complex circuitry to compensate for reduced linearity. The power amplifier chips used in the present invention in the linear range typically have a low output power of one watt or below. Moreover, the invention proposes installing a power amplifier chip of this type at the feed point of each element of a multi-element antenna array. Thus, the output power of the antenna system as a whole may be multiplied by the number of elements utilized in the array while maintaining linearity.
Furthermore, the present invention does not require relatively expensive high power combiners, since the signals are combined in free space (at the far field) at the remote or terminal location via electromagnetic waves. Thus, the proposed system uses low power combining avoiding otherwise conventional combining costs. Also, in tower applications, the system of the invention eliminates the power loss problems associated with the relatively long cable which conventionally connects the amplifiers in the base station equipment with the tower-mounted antenna equipment, i.e., by eliminating the usual concerns with power loss in the cable and contributing to a lesser power requirement at the antenna elements. Thus, by placing the amplifiers close to the antenna elements, amplification is accomplished after cable or other transmission line losses usually experienced in such systems. This may further decrease the need for special low loss cables, thus further reducing overall system costs.